The main oilfields in China such as Daqing oilfield, Shengli oilfield, and Liaohe oilfield etc. have entered the late stage of secondary oil recovery having high water cut and extra-high water cut. In order to improve the ultimate recovery of the existing oilfields, it is desirable to exploit tertiary oil recovery technique. Combined chemical flooding technique is a new tertiary oil recovery technique developed in 1980's, which combines alkali, surfactant and polymer and takes advantage of the synergistic effect between each agent. Such technique not only can substantially reduce the interfacial tension of oil-water and increase the microscopic oil displacement efficiency, but also can increase the viscosity of the displacing fluid so as to have relatively high swept efficiency, thereby substantially improving the recovery efficiency. In practice, due to the use of the strong alkali and the too high dosage, sonic disadvantages are observed, such as the alkali consumption and scaling, damage of formation, decrease of the viscosity of displacing fluid, and the severe oil/water emulsification, which significantly increases the difficulty in the treatment of produced liquid and cost, affects the lifting technique in oil production and causes severe facility corrosion. Therefore, alkali, especially strong alkali should be avoided in combined flooding. The systems with weak alkali or without alkali have higher viscosity and elasticity than that of strong alkali ASP flooding, which can reduce the amount of polymer and increase the sweep efficiency. Moreover, the formulation on the spot, devices for injection and processing of such systems are easier than those of ASP systems, which can reduce the cost. Therefore, it is the developing trend of the combined flooding to use weak alkali and alkali-free systems, to which the oilfield developers have paid increasing attention. However, at present, the biggest constraint for developing weak alkali/alkali-free systems is the development of high efficiency surfactants. Compared with strong alkali ASP flooding, weak alkali/alkali-free combined flooding weakens the effect of alkali, thus making it more difficult to reach ultra-low interfacial tension. Presently, the surfactants in research remain at the stage of laboratory screening with high cost, which cannot satisfy the need of practical production.
Betaine surfactants are amphoteric surfactants. Due to their chelating effect on metal ions, most of the betaine surfactants can be used for oil displacement of high salinity and high temperature reservoirs, and are capable of substantially reducing the chromatographic separation effect which occurs when nonionic surfactant is combined with anionic surfactant. The betaine surfactants mainly include carboxyl betaine type and sulfo-betaine type. The use of sulfo-betaine amphoteric surfactants in tertiary oil recovery has been reported in US patents. Such surfactants solutions prepared by high salinity water with high divalent ion content are very effective in reducing the oil-water interfacial tension, and have good emulsification and solubilization properties. Darling Petroleum College has successfully developed a new carboxyl betaine BS13 surfactant system. The results of in-lab displacing experiment show that BS13 oil-displacing system has better oil-displacing effect than strong alkali ASP system. However, the synthetic routes of such surfactant BS13 is complicated, with high cost, and the production craft needs to be improved.